1. Field of the Invention
This invention relates to a semiconductor optical guided-wave device such as, for example, an optical switch and optical modulator and to a method of producing such device.
2. Description of the Prior Art
Recently, an optical switch and optical modulator have appeared as one of key elements for high-speed optical communication systems and optical information processing systems of the future. As a result, many research and developmental activities have positively progressed. Under such a circumstance, a known device to be used for this purpose, may have a dielectric material made of LiNbO.sub.3 or the like and a semiconductor made of GaAs, InP or the like.
Recent demands for a semiconductor optical switch and optical modulator are that they be integrated with other optical elements including optical amplifier and/or electronic circuits including field effect transistors (FET), these semiconductor devices should be easily compactized and multi-channeled. In order to fill these demands, considered from the viewpoint of the above application fields, they are required to be operable at a high level of speed and low levels of loss. Power consumption should be low and the voltage should be easily applied. The semiconductor device should be adapted to be highly integrated in an easy manner.
As a physical effect to be applied for a semiconductor optical switch or optical modulator, a band filling effect or free carrier plasma effect, the refractive index is varied with the application of an electric current. An electrooptic effect of the refractive index is varied accompanied with the application of a voltage. A quantum confinement stark effect (QCSE), that is the refractive index, is varied accompanied with the shift of an excitonic absorption peak when a voltage is applied to a multiple quantum well, or the like. Many attempts and investigations are using these effects, as described above, with an optical switch or optical modulator utilizing the band filling effect or free carrier plasma effect due to the application of an injection electric current. Problems have been pointed out because the operating speed is low and the power consumption is large. On the other hand, with an optical switch or monitor that utilizing the QCSE due to the multiple quantum well structure, there is a problem because it is naturally difficult to be operate at a low level of losses.
On the other hand, an optical switch or optical modulator utilizing the electrooptic effect, becomes longer in length than those utilizing the other effects. However, high-speed operation and power saving operation as well as low-loss operation advantageously become possible. Referring to the low-loss operation requirement, an optical waveguide having a loss as low as 0.15 dB/cm with a wavelength of 1.52 .mu.m was realized utilizing a GaAs/AlGaAs system semiconductor as was reported by E. Kapon and others in the journal of "Applied Physics Letters" Vol. 50, No. 23, pp. 1628 to 1630, 1987.
The possibility of realizing such a low-loss optical waveguide, as described above, is because the bandgap wavelengths of GaAs and AlGaAs are sufficiently short as compared with the operating wavelengths of 1.3 and 1.5 .mu.m, respectively. In this case, the dependence of the electrooptic effect on the wavelength is small, As a result, even if the operating wavelength is apart from the bandgap wavelength, the change in refractive index is not so different from that when it is in the vicinity of the bandgap wavelength. Besides, the low-loss optical waveguide reported, as described above, was formed by a wet chemical etching method.
In this case, however, the wet chemical etching method applied for forming the above-mentioned optical waveguide makes it possible to provide a low-loss operation, but is not preferable to form it over a large surface area with a good reproducibility. This means that it is difficult to apply such an optical waveguide for an integration of optical devices as well as for mass-production of discrete optical devices.
Application of a dry etching method instead of the wet chemical etching method makes it possible to form a fine optical waveguide over a large surface area with a good reproducibility. In this case, however, with the conventional dry etching technology, it is unavoidable that a slight irregularity is formed on an etched bottom surface and/or etched side surface. As a result, the scattering loss of a propagation light will be caused by the slight irregularity thus formed, thus the reduction of losses being limited. In addition, the dry etching may induce a damage to the etched bottom surface, which gives a loss to the propagation light. Consequently, it is difficult for a semiconductor optical waveguide prepared by the dry etching technique to be made smaller in loss than that prepared by the wet chemical etching technique.
Furthermore, in case of using InP system material, such a problem has been pointed out that the dry etching technology itself has not yet been established satisfactorily. This is because when an InP system material is dry-etched using a chlorine gas or a mixture of a chlorine gas and other gases which are generally used in the dry etching of compound semiconductor material such as, for example, GaAs, a chemically stable chloride InCl.sub.3 is generated to prevent the etching from being progressed. As a result, it is difficult to obtain a practical and acceptable etching speed. In order to restrain the generation of such a chemically stable chloride of indium, in dry-etching the InP system material, it can be considered that the chemical reactivity is restrained thereby etching the material mainly by a physical sputtering process. In this case, however, the damage to an etched bottom surface becomes large, being undesirable.
Recently, such a technique that uses a methane system gas has been introduced in dry-etching the InP system material has been proposed. However, if methane system gas is used for this purpose, a contamination of semiconductor material by carbon becomes unavoidable, thus a problem to be solved remains.
As explained above, optical guided-wave devices such as the semiconductor optical switch and optical modulator utilizing the electrooptic effect are hopeful. However, from the view point that the dry etching technology itself has not yet been fully established, a satisfactory technology to produce a fine, low-loss optical waveguide capable of use for a large-scale integration of optical devices or massproduction of single optical devices has not yet been established.
Thus, an object of this invention is to provide a semiconductor optical guided-wave device which is fine instructure and low in loss as well as adapted to meet mass-production and integration requirements, and its production method.